Norms have become a key concept in Translation Studies, at least since Toury (1980). But there is still disagreement about how best to define them, and also how to study them. Basically, there are two senses of the concept “norm”: one is descriptive and weakly explanatory, and the other is causal or prescriptive and more strongly explanatory. Studying norms in the causal sense means looking for plausible links between observed regularities and evidence of normative force: this may be found in belief statements, in criticism of norm-breaking, or in norm statements. Norms are explanatory hypotheses, and can be tested in various ways.